Linear accelerator in medicine

A linear accelerator

Linear accelerator is apparatus, whis is used in medicine for radiotherapy i. e for the irradiation of malignant tumor diseases.

It works on the principle of electron acceleration. It is used to accelerate particles along a linear straight path of electric field. It is static or it can change. Linear accelerators are separated to electrostatic or high frequency. Electrostatic linear accelerators are futher divided by type of high voltage source. High- frequency linear accelerators can be divided by type of waves. A linear accelerators has these components like other accelerator. The components are:

• the source of accelerated particles
• target/accommodating union
• detectors

Types of linear accelerators

1. electrostatic linear accelerator

• cascade generator( Cockroft-Walton generator)
  • Wande Graff generator
  • Tandem generator

1. induction accelerator
2. rezonant hogh frequency accelerator
   • with electrodes
   • with a carrier wave

Electrostatic linear accelerator

Electrostatic linear accelerator uses a potentil difference between anode and cathode. A discharge lamp is the siplest example of such an accelerator. It is composed of a high voltage source, a hollow accelerator tube and a target on which fall the accelerated particles.

A ion source emits the particles into a system of metal cylindrical electrodes. A gradually increasing high voltage(U)is located between the electrodes. Particles with charge q is speeded up by electrostatic field on energy E.

E=q( U₁+U₂+U₃ ...+U_n)

Each gap between the electrodes orients the flying particle into the narrow bundle, which hits the target. It is source of X-ray.

Cockroft-Walton generator

The basic component is a voltage multiplier. High voltage reaches here by multiple alternating voltage. It is obteined by transformer. The gained energy comes up to 4 MeV. Aplication: injector for bigger accelerators.

Van de Graff generator

The principle is a transmission of a positive charge from external source to inside a metal ball using a moving belt. The charge is then taken to the surface of ball and the field inside remains zero. A discharged part of the belt is coming back again to charge. A charge energy will increase up to 10 MeV.

A maximal voltage is theoretical unlimited, but it is limited by a breakdown voltage of the surrounding gas. The solution of this problems are high pressueres around or a location of generator to special gases.

Tandem accelerator

It is just modification of CW or VdG generator.

The induction accelerator

A transformator is important component because a the bundle of particles goes through it, which acts as electric current. The one in the transformer induces a field thanks this, the particles are accelerated. It is used for a sterilization and TOKAMAK.

Resonant high-frequency accelerator

With electrodes

It is an effective way of accelerating of charged particles without using of enormous high voltage. The particle are emited form a high-frequency source to the acceleration system of cylindrical electrodes. Odd electrode cylinders are connected to one pole, even electrode cylinders are connected to the other pole.

U(t)=Ucosωt=Ucos2πft

A proper synchronization must be selected for achieving acceleration between individual quantities. The lehgth of cylindrical electrodes must be increased with increasing speed of particles. The particles are accelereted in the spaces between the cylinders.

With the carrier wave

By using plated cavity resonators(cavities) as a waveguide-an electromagnetic carrier wave passes through them. It occurs to great acceleration on short way. It exist two types of this accelerator: with a gradual wave and with a carrier wave.

• with a carrier wave: It works for acceleration of electrons. It often uses a breaking attachments, which armed to phase velocity of an electromagnetic wave is exceeded a the speed of light.
• with a carrier wave: A high-frequency wave will pass on the fast track and in the end it is reflected back. This creates the carrier wave with very high forced pole.

References

1. DOLEŽAL, Zdeněk. Urychlovače <http://www-ucjf.troja.mff.cuni.cz/dolezal/teach/accel/urych01_t.pdf> [online].
2. ↑ RICHTROVÁ, Michaela. Urychlovače elementárních částic <https://is.muni.cz/th/cbysw/bc.pdf?so=nx>[online].